Hydraulic cements are used in concrete and most masonry mortars. The principal ingredients of Portland cement, the best known hydraulic cement, are tricalcium silicate, dicalcium silicate, and calcium aluminum ferrite. These ingredients, when in a finely ground state and mixed with water, react to form a very hard material with an intermeshed crystalline structure.
The most common method of preparing Portland cement is by introducing finely ground lime and silica into a rotary kiln and heating the material to at least the point of incipient fusion (which is about 2325.degree. F.) in the presence of alumina or iron oxide fluxes. The kiln itself is an elongated cylinder with its axis inclined slightly from the horizontal. The kiln is provided with a burner at its lower end. The dry ingredients are added to the upper end of the kiln and, owing to the slant of the kiln and the rotation of the kiln, the material gradually moves from a cool zone at the upper end to the fusion zone. In the hotter region of the kiln a reaction takes place to form new compounds which fuse and partially melt. This material when cooled becomes the well-known cement clinker. The clinker is then finely ground to create Portland cement.
Two of the major problems inherent in this process for preparing cement are the handling and disposing of the large quantity of dust generated in the process. The dust is caused by the powdery nature of the starting materials and the intermixing caused by the rotating kiln. Kiln dust, which can be as much as 40 to 50% of the starting ingredients, can be recycled to some extent, but total recycling is not possible because of an undesirable build-up in the alkali content of the dust. If the alkali content becomes too high, the final product will lose its cementitious properties. So in most plants where recycling is used, an alkali by-pass is required in the kiln off-take gas flues to bleed off a portion of the dust and alkalies and obtain control of the alkali content of the kiln feed. It will be appreciated then that alkali bearing kiln dust is a major environmental pollution problem for this industry. In addition, the dust loss problem is expensive because it decreases the production and material utilization efficiency of the plant.